1. Field of the Invention
The invention concerns a method for pressure- and flow-preventive fixing of pipes, for example casings and liners and possibly accompanying equipment, in a well when drilling the well. The method may also be employed in a well, for example a completed well, in order to place one or several pressure- and flow-preventive barriers in one or several cavities/voids of the well, preferably annuli, wherein at least one adjoining pipe of the cavity/void/annulus is leaking.
The method according to the invention has developed mainly as is a consequence of a large and increasing need existing among authorities and industry, primarily the petroleum industry, to improve and eventually replace prior art methods for fixing casings in a well, prior art methods being encumbered with a series of severe problems and disadvantages, and cement being the primary prior art means for fixing casings and liners in the well.
2. Prior Art
In connection with the drilling of a well, for example a petroleum well, and after having drilled a borehole down to a desired depth in the subsurface, it is customary to case the borehole with pipe(s). Usually the well consists of several such boreholes, or hole sections, that sectionally and consecutively run with diminishing hole diameter into the subsurface. It is therefore customary to provide the consecutive hole sections with casings of sectionally diminishing pipe diameters, wherein one casing size is placed within the preceding casing size etc. Each casing size usually runs up to, and is connected to, the wellhead of the well. So-called liners represent one exception to this which, on the other hand, do not run up to the wellhead of the well, and liners usually being employed to case one or several of the deepest hole sections of the well. Such liners are usually fixedly cemented within and to a lower part of a preceding casing in such a way that the upper part of the liner overlaps the lower part of the preceding casing only.
Most casings, including liners, are fixed by cementing to the relevant borehole wall and usually also to the preceding casing. In this context it is customary first to compute the amount of external annular volume of the pertinent casing to be filled with cement slurry, thereafter placing into said annulus/annuli a volume of cement slurry corresponding to at least that of the computed annular volume. With the exception of liners, cementing of most casing sizes is carried out by pumping said volume of cement slurry down through the pertinent casing, thereafter forcing the cement slurry out/up into the annulus between the pertinent casing and the hole wall of the well and, eventually, usually also up into at least a lower part of the annulus between the pertinent casing and the preceding casing. The cement slurry may be pumped in one or several stages, and into all or parts of the pertinent casing length, after which the cement slurry in principle shall harden into cement.
In the well, in order to avoid mixing, and thereby contaminating, the cement slurry with other liquids, usually drilling fluid, it is customary to place the cement slurry between two movable plugs, so-called wiper plugs, placed in the particular casing in order to facilitate the displacement of the cement slurry. The lower and foremost of said plugs is a leading plug, while the upper and hindmost plug is a trailing plug. Subsequently, and by means of pumping, the cement slurry and said plugs are displaced down through said casing. The leading plug is arranged with a through-going hole which is covered by a diaphragm (a membrane), while the trailing plug usually is a solid and is substantially stronger than the leading plug. By means of a fluid displacement column, usually a column of drilling fluid, placed on top of said trailing plug and arranged with necessary pumping equipment, the cement slurry and said plugs are subsequently pumped dorm through the casing until the leading plug is brought into contact with, and is arrested by, an associated seat or stopping device at the bottom of the casing. Subsequently the pump pressure is sufficiently increased for said diaphragm to rupture, after which the cement slurry is pumped through said hole in the leading plug and is further displaced out/up into said annulus/annuli. The pumping of cement slurry down through the casing continues until the trailing plug is brought into contact with, and is arrested by, the leading plug. The displacement of cement slurry out/up into said annulus/annuli is thereby completed, but a sufficiently large liquid pressure is maintained in the overlying fluid displacement column for the cement slurry to harden without introducing movements in the cement slurry during the curing process.
In connection with fixedly cementing a liner in a well, however, a cementing pipe must be connected between cementing equipment at the surface of the well, for example at/on a drilling rig, and a lower part of said liner. Usually, such a cementing pipe is comprised by a string consisting of connected drill pipes, the lower end part of the drill string being provided with an open and suitably adapted pipe, a so-called stinger, the stinger first being introduced into the well and being connected to a valve device located in the lower part of said liner. Analogous to the above-described method, cement slurry and associated leading- and trailing plugs may subsequently be pumped down through the cementing pipe and onwards to said valve device, after which the cement slurry is displaced out/up into the external annulus of the liner.
In the hardened condition, the cement constitutes a fixed mass which, among other things, shall function as a pressure- and flow-preventive barrier in said annulus/annuli of the well. In the event of potential fluid pressure differentials existing in the well, the cement shall prevent formation fluids from flowing between various formation layers and/or prevent formation fluids from flowing further upward in the well and possibly entirely to the surface. Also, the cement shall maintain the casings fixed to the borehole wall of the well and usually also within and to a preceding casing. For example, a surface casing of a well will largely support the weight of the other and smaller casing sizes of the well and also a wellhead or a blow-out preventer (“BOP”), and in this regard it is therefore necessary to establish a shear sustainable bond between the surface casing and the surrounding rocks, and in such a way that said loads may be transferred to the surrounding rocks. Thus, the shear sustainable and load transferring bond often consists of cement. Moreover, and upon commencing the drilling of the subsequent hole section, cement underlying and surrounding a casing shoe may contribute to stabilise a potentially fractured or unconsolidated rock in the hole wall of the well. This stabilisation of said hole wall contributes to prevent or reduce the falling of rock fragments from the hole wall of said well region and into the subsequent hole section while the drilling thereof is carried out.
In order to drill a well down to a drilling objective, for example an oil/gas reservoir, usually it is of absolute necessity to place in the annulus of the well, a pressure- and flow-preventive mass, for example cement and/or a pressure- and flow-preventive device, potentially a sealing arrangement, for example a mechanical packer. This particularly applies when drilling deep wells and/or when drilling wells down into subsurface layers wherein large fluid overpressures exist, simplistically denoted as overpressure. An overpressure exists if the pores of a subsurface rock layer are exposed to a fluid pressure exceeding the liquid pressure which otherwise would exist if the layer was exposed to a normal hydrostatic pressure gradient from the surface and down to the subsurface layer of interest.
Upon drilling down through the various subsurface layers, a drilling fluid with a specific gravity, and thereby a hydrostatic liquid pressure, which is arranged to counteract the fluid pressure in the rock pores being penetrated, is used in the borehole. This is done to prevent a potential and undesired inflow of formation fluids into the well. When, during drilling at ambient conditions, a normal hydrostatic gradient exists in the subsurface pore fluids, normally the pressure gradient which may be observed in water-filled upper layers of the subsurface, said hydrostatic pore fluid pressure may be counteracted by arranging the drilling fluid with a slightly larger specific gravity/pressure gradient.
The various subsurface formation layers may also exhibit different properties of strength, wherein the rock strength largely may be related to lithological composition, particle distribution, particle cementation and degree of compaction of the subject rock. Generally, the rock strength increases with increasing depth into the subsurface. This implies that rocks being penetrated by a well, may be exposed to, and may resist, a gradually increasing fluid pressure without fracturing being initiated in the rocks. A further increase of said fluid pressure will, however, result in fracturing of one or several of the penetrated rocks, this fracturing pressure commonly being denoted as the fracturing pressure of the subject rock(s), and the fracturing pressure commonly being recalculated, and expressed in terms of, an equivalent fracture gradient of the subject rock(s).
During drilling, upon approaching one or several formation layers with expected overpressures), the specific gravity/pressure gradient of the drilling fluid is increased to an extent necessary to withstand said overpressure(s). Thus, potentially overpressured formation fluids are prevented from flowing into the well upon drilling into, potentially after having drilled into, said layer(s). If said increase in the pressure gradient of the drilling fluid exceeds the fracture gradient of one or more of the penetrated rocks, the rocks(s) will be fractured and fractures develop in the rock(s). Then, drilling fluid may flow unobstructedly out (leak) from the well and into the fractures, thereby causing the height of the drilling fluid column, and thus the liquid pressure in the liquid column, to be lowered. By so doing, the formation pressure barrier brought about by the drilling fluid pressure exerted in the well is impaired, and this results in the establishment of an undesired, and potentially very dangerous, situation in the well. In order to prevent such fracturing it is often absolutely necessary to isolate the penetrated formation layers from pressures that may fracture the rocks contained therein. As mentioned, such a fracturing pressure may be exerted by the pressure, of the drilling fluid column, but the fracturing pressure may also be exerted by the overpressured formation fluids of other formation layers, usually deeper formation layers, which are being penetrated by the well during drilling.
Moreover, and pertaining to an open hole-section, it is the rock(s) of the shallowest part of the section, immediately underlying the casing shoe of the preceding casing, that generally, but not necessarily, is/are the weakest by strength, and thus being the one(s) which may first be fractured. After having started the drilling of a new hole section in a well, it is for this reason common practise to undertake a so-called formation strength test of the shallowest rocks in said hole section. Such a formation strength test is usually carried out immediately after having drilled the uppermost rocks along a 5-10 metre hole length of the new hole section. For example, the formation strength test may consist in supplying said rocks with drilling fluid under a gradually increasing liquid pressure, and increasing the liquid pressure until an incipient fracturing of, and an accompanying leakage of drilling fluid into, the rocks is observed, which determines the fracturing pressure/fracture gradient of the rocks. In the petroleum industry such a formation strength test is usually called a “leak-off test”. In another commonly occurring formation strength test, a so-called formation integrity test, said rocks are also supplied with drilling fluid under a gradually increasing liquid pressure, limiting however the fluid pressure increase to a predefined maximum liquid pressure, and where this liquid pressure is considered to be the maximum required drilling fluid pressure to be applied for the new hole section to be drilled down to the desired drilling depths. This maximum liquid pressure is usually smaller than the fracturing pressure of said rocks, thus not fracturing the rocks during this formation strength test. Therefore, a formation integrity test is usually gentler on said rocks and the subsequent drilling operations than a fracturing test. Such formation strength tests therefore provide a good indication as to the magnitude of liquid pressure, or magnitude of the liquid pressure gradient, whereby the drilling fluid may be arranged during the drilling of a hole section in order to avoid fracturing of the accompanying rocks. Said maximum liquid pressure/liquid pressure gradient also limits the further drilling of a hole section to end at a depth at which the fluid pressure of a formation layer approaches said liquid pressure/liquid pressure gradient.
Cementing is also employed as a corrective method to prevent/reduce undesired inflow, and thereby also undesired pressure build-up, of a fluid in one or several regions of a well, including undesired fluid inflow through one or several leaking casings surrounding uncemented annuli of the well, the annulus/annuli possibly extending entirely up to the wellhead of the well. The method consists in injecting cement slurry, possibly with the addition of plasticizing agents, gelling agents, stabilisers or other additives, into a relatively short annular interval covering said inflow region(s), whereupon the cement slurry or agent hardens or sets in such a way that it forms a pressure- and flow-preventive barrier which, in principle, shall prevent/reduce such fluid inflows.